1. Field of the Invention
The present invention is broadly concerned with a method of immunizing cats against shedding of Toxoplasma oocysts in their fecal elimination processes. More particularly, it is concerned with such a method in which immunization can be accomplished without the necessity of incurring initial oocyst shedding as has heretofore been thought to be essential.
2. Description of the Prior Art
Cats and people are biologically close enough for them to be able to suffer from some of the same diseases and parasites. Some of these ailments are well known--rabies, because it is so dramatic and terrible, and fleas, because they are so common, are two familiar examples. Less familiar is Toxoplasmosis, an animal disease that can be transmitted to man. It is common to all domestic animals, including barnyard species, and, when present, is transmitted through cat feces and meat.
Any direct contact with cat feces increases the likelihood of contagion. Cattle and sheep can become infected by ingesting contaminated soil while grazing. Birds may become infected when eating seeds on the ground or from eating contaminated earthworms. However, a domestic cat, after stalking a mouse or bird, may disseminate the disease. Although all these infected animals may be means of transmitting disease, more often than not there is no sign of illness.
Toxoplasmosis is a parasitic disease, and research has indicated that the parasite has a complicated life cycle which spreads the disease through many animals. Oocysts (egg spores) are shed in the feces of domestic cats and certain types of wild cats. Oocysts are then spread by contact with the feces. Flies and cockroaches, which eat feces, can serve as transport agents, contaminating animals which do not directly encounter the cat feces. Mice and birds can be infected either from transport agents or through direct contact and can then spread the infection to animals which prey on them. Humans can be infected by eating raw or rare meats, or by direct contact with infected cat feces.
Toxoplasma infections are quite prevalent, with one-quarter to one-half of the adults in the United States and elsewhere asymptomatically infected. While the presence of Toxoplasma infections has long been known, little was discovered about the transmission of Toxoplasma until the late 1930's and 1940's when Toxoplasma was found in newborn babies in the U.S. However, the life cycle of Toxoplasma, and the central role played therein by cats, has now been conclusively established.
The spectrum of human disease due to Toxoplasma was characterized by a combination of serologic, immunologic and epidemiological studies, and by isolation of Toxoplasma. In the acute infection where cells are destroyed by rapidly proliferating organisms, there may occur fever, pneumonia, an inflammation of the heart muscle, liver and skin (rash). Toward the end of this period or following a subclinical acute infection, localized or generalized swelling of lymph nodes is observed, especially in women. In newborns infected in utero, a subacute disease picture is typical. In addition to the symptoms of acute Toxoplasmosis mentioned above, meningoencephalitis ("Brain fever"), often with hydrocephalus ("water on the brain"), and retinochoroiditis (intraocular inflammation) are important. Most of the mothers who have given birth to infected babies had infections without symptoms.
Thus, Toxoplasmosis deserves special attention because of the serious danger it raises for the unborn human baby. A pregnant woman may have the infection and unknowingly infect the fetus. If not diagnosed and treated in time, her child may be born with permanent brain and eye damage. For this reason, efforts to prevent infection during pregnancy are important.
Inasmuch as domestic cats are an important carrier of Toxoplasma and shed infectious oocysts in their feces, attempts have been made in the past to immunize such cats against oocyst shedding. Generally speaking, prior successful immunizations have required primary infection of cats with Toxoplasma, followed by the usual oocyst shedding and a buildup of immunity. However, this manner of immunization generates the very phenomenon sought to be avoided, i.e., oocyst shedding, and as such is deemed deficient. This is especially the case when it is considered that infectious oocysts tend to remain active for a period of months up to a year and a half.
Therefore, there is a decided need in the art for an effective method of immunizing cats against oocyst shedding without incurring oocyst shedding as a result of vaccination.